Nozzle of a turbomachine provided with chevrons with a non-axisymmetric inner face

ABSTRACT

A nozzle of a turbomachine is provided with chevrons with a non-axisymmetric inner face. A cover for the nozzle of longitudinal axis L-L includes an internal wall having a section with a predetermined abscissa on the L-L axis defining, on the wall, a neck line. The cover has, downstream from the abscissa, indentations in the trailing edge of the cover delimiting chevrons distributed in the circumferential direction, in which, on the cover, from the abscissa the internal wall moves away radially towards the outside of the upstream tangent, to the top of an indentation, the outer wall of the cover moves towards the upstream tangent to the top of an indentation, and the internal wall moves away radially towards the inside of the upstream tangent, to the tip of a chevron.

TECHNICAL FIELD

The present invention relates to the general field of reduction of thejet noise at the outlet of a turbine engine nozzle.

In particular, the present invention concerns the definition of theshape of a turbine engine nozzle which has, at its downstream end, atrailing edge comprising chevrons in order to reduce the jet noise.

PRIOR ART

It is known that the jet at the outlet of a turbine engine nozzleencounters at least one other gaseous flow. Thus:

-   -   when the turbine engine has a single flow, this flow comes into        contact with the external air bypassing the nozzle; and    -   when the turbine engine has a double flow, the primary flow (or        hot flow) and the secondary flow (or cold flow) come into        contact with one another, but also with the external air.

The speed difference between the jet at the nozzle outlet and the othergaseous flow or flows which it encounters creates fluid shearspenetrating between these gaseous flows, which causes noise, commonlydesignated as “jet noise”. This “jet noise” is a broadband noise whichis particularly inconvenient during the aeroplane take-off phase.

The use of chevrons placed in a ring at the outlet of the nozzle is aknown means for substantially reducing the low-frequency components ofthis noise. The patent application EP1873389A1 describes chevrons whichmake it possible to reduce the turbulent intensity of large vorticesconstituting the major sources of noise. The low-frequency noise is thensubstantially decreased.

However, the incorporation of anti-noise chevrons in the region of thetrailing edge of the nozzle is not entirely satisfactory. In fact, apronounced inclination of the chevrons towards the outlet jet isgenerally necessary in order to obtain an acceptable reduction of thejet noise, which leads to a deterioration of performance levels, inparticular the thrust, of the turbine engine at its different operatingspeeds. In addition, by decreasing the effective discharge cross-sectionof the internal gaseous flow of the nozzle, the inclination of thechevrons reduces the thrust ratio of the turbine engine between thetake-off speed and the cruising speed. This results in a loss ofoperability of the turbine engine. This problem is particularlysignificant in the case of a convergent-divergent nozzle designed tooptimise this operability.

In the patent application FR1351152, the applicant has proposed anarrangement where the cross-section of the nozzle after the neck, whilstremaining circular, forms a divergent part into which the indentationsof the cowl open, the chevrons retaining a convergent part relative tothe neck in order to retain their effectiveness in terms of noisereduction. Moreover, the divergent part, which is open in certainsectors due to the indentations, makes it possible to obtain aneffective discharge cross-section which is sufficient for maintainingthe operational performance levels of the nozzle. However, this designdoes not allow sufficient degrees of freedom in order to treat all casesand has drawbacks for the internal flow, creating a possiblerecirculation zone in the zone where the chevrons curve in order tobecome convergent again.

The present invention proposes an alternative in order to remedy thesedrawbacks and, in particular, to define the shape of a nozzle whichmakes it possible to achieve a greater effectiveness over all of thethree criteria of performance levels, operability, and acoustics.

DESCRIPTION OF THE INVENTION

The invention relates to a cowl for a nozzle having a longitudinal axisL-L delimited radially towards the exterior by its external wall andradially towards the interior by its internal wall, the internal wall ofthe cowl having a cross-section with a determined abscissa X0 on theaxis L-L defining a neck line on the wall, the upstream internal wallapproaching the axis L-L by moving towards said cross-section, and saidinternal wall converging towards the axis L-L upstream of saidcross-section and having, in any axial half-plane, a defined upstreamtangent at its intersection with the neck line.

Said cowl has, downstream of said determined abscissa X0, indentationsin the trailing edge of the cowl which delimit chevrons distributed inthe circumferential direction, and is characterised in that, in thedownstream direction from said determined abscissa X0:

-   -   the internal wall of the cowl diverges radially towards the        exterior, in a first axial half-plane passing through the top of        an indentation, from the upstream tangent on the point of the        neck line in this half-plane,    -   the external wall of the cowl moves closer, in said first axial        half-plane, to the upstream tangent on the point of the neck        line in said first axial half-plane, and    -   the internal wall of the cowl diverges radially towards the        interior, in a second axial half-plane passing through the tip        of a chevron, from the upstream tangent on the point of the neck        line in said second axial half-plane.

In the present application, the terms “upstream” and “downstream” referto the direction of the gaseous flows in a turbine engine or the nozzlethereof. A nozzle is defined as comprising a cowl of which the internalwall delimits a conduit into which an internal gaseous flow passes. Acentral body can be placed inside the cowl. In this case the conduit hasan annular shape. A nozzle is said to be convergent (or divergentrespectively) when the distance of the internal wall of the cowl fromthe central body or from the axis of the nozzle decreases (or increasesrespectively) in the downstream direction. An axial plane is understoodto be a plane passing through the axis of revolution L-L, and an axialhalf-plane is the part of the axial plane extending on only one side ofthe axis L-L.

In addition, in the present application the cowl has a thickness, and istherefore of the double-wall type and comprises an external wall aroundwhich flows an external gaseous flow, in the same direction as theinternal gaseous flow. This external gaseous flow may be a secondaryflow of the turbine engine, or the ambient air in which the aircraftmoves.

The neck cross-section serves for reference. In addition, at thislocation of minimal cross-section the mean direction of the gaseous flowin the nozzle is substantially parallel to the walls. Therefore, thetangent of the internal wall of the cowl to the neck gives anapproximation of this direction of the gaseous flow at thiscross-section. The three conditions defined in the invention thereforeperform the following functions:

-   -   constructing indentations which cause the internal flow to        diverge,    -   guiding the external flow in the region of the indentations in        order to promote the mixing of the flows,    -   constructing re-entrant chevrons in the internal flow,    -   and guiding the external flow in the region of the tips in order        to promote the mixing of the flows.

On the one hand, the convergent design of the chevrons makes it possibleto optimise them for reduction of the noise. On the other hand, the topsof the indentations can diverge from the axis regardless of the shape ofthe cross-section in the region of the chevrons, without taking accountfor example of the structural stresses of thickness of the cowl in theregion of the chevrons. This makes it possible to obtain a largerequivalent discharge cross-section than in the previous solutions. Thisdischarge cross-section is an important parameter in order to ensure theperformance levels of the nozzle in various operating conditions, forexample in order not to restrict the flow rate coefficient. In a way,the invention makes it possible to separate the geometric designparameters of the nozzle between those governing the acousticperformance levels and those governing the operational performancelevels. The fact that the divergence of the external wall decreasesconstantly towards the top of the indentations makes it possible todecrease the separation of the external flow in this zone and thuscontributes to the operational effectiveness of the nozzle. Anotheradvantage of the invention is that it makes it possible to eliminate thepart of maximum internal cross-section of the nozzle in the chevrons,corresponding to a possible separation zone.

The lines defining the internal wall of the cowl upstream and downstreamof the neck line in any axial half-plane preferably have the sametangent on this neck line. This avoids disruptions in the internal flow.

The line defining the internal wall of the cowl downstream of saiddetermined abscissa X0 in an axial half-plane passing through the tip ofa chevron is preferably concave when viewed from the axis L-L, with aview to improving the re-entrant nature of the chevron and avoidingrecirculation zones in the internal flow.

The line defining the internal wall of the cowl downstream of saiddetermined abscissa X0 in an axial half-plane passing through the top ofan indentation is preferably convex when viewed from the axis L-L. Thisaccords with the fact that the internal shape of the cowl forces theinternal flow to diverge in this zone and to meet the external flowopposite a confluence where the two flows would be substantiallyparallel.

The line defining the external wall of the cowl downstream of saiddetermined abscissa X0 in an axial half-plane passing through the top ofan indentation is also preferably concave when viewed from the axis L-L.This condition reinforces the condition on the overall convergence ofthe external wall in an indentation in order to indicate that thedirection imposed by the external wall at the top of the indentation onthe external flow is re-entrant with respect to the mean direction ofthe internal flow.

Advantageously, the lines defining the external wall of the cowl in anyaxial half-plane do not have a turning point downstream of saiddetermined abscissa X0 of the neck line, in order to limit the risks ofseparation in the external flow.

Advantageously, the lines defining the internal wall of the cowl in anyaxial half-plane do not have a turning point downstream of the abscissaX0 of the neck line. This condition, in a similar manner to that of theexternal wall, limits the separation of the flow inside the nozzle.

Advantageously, the thickness of the cowl on its trailing edge issubstantially constant. In fact, the thickness of the cowl isadvantageously reduced to the minimum permitted by structuralconsiderations at the tip of the chevrons in order to ensure theconfluence of the two flows. It is useful to maintain this minimumthickness over all of the trailing edge in order to avoid recirculationzones due to a truncated trailing edge effect. In this way, convergentsectors can be defined around chevrons and divergent sectors can bedefined around indentations, thereby organising the confluence betweenthe external and internal gaseous flows to the cowl of the nozzle.

The invention also relates to a nozzle for a turbine engine having alongitudinal axis L-L and comprising a cowl as defined previously.

The invention also relates to a nozzle for a turbine engine having alongitudinal axis L-L and comprising a cowl as defined previously and acentral body of revolution about the axis L-L.

The features of the cowl advantageously make it possible to obtainproperties which are remarkable in terms of the passage cross-section ofthe gaseous flow. Thus it is possible to obtain such a nozzle which hasa central body and in which the radial passage cross-section at a pointon a wall of the cowl is defined as the square of the radial distance ofsaid point from the axis L-L minus the square of the radial distance ofthe axis L-L from the point of the wall of the central body situated onthe same radius when said central body is present at the abscissa on theaxis L-L of said point, and by the square of the radial distance of saidpoint from the axis L-L when this is not the case, said nozzle beingcharacterised in that, in the downstream direction from said determinedabscissa of the neck line, the radial passage cross-section of theinternal wall of the cowl increases in an axial half-plane passingthrough the top of an indentation.

Thiscondition corresponds to the fact that the enlargement of the radialpassage cross-section in the region of the indentations aims to increasethe effective passage cross-section of the nozzle, or at the very leastto preserve this effective passage cross-section, in spite of theconstriction in the region of chevrons, with a view to increasing theoperational performance levels of the nozzle.

More generally, it is also possible to define a nozzle comprising a cowlwhich is delimited radially towards the exterior by its external walland, radially towards the interior, by its internal wall,

-   -   the internal wall of the cowl having a cross-section with a        determined abscissa X0 on the axis L-L defining on the wall a        neck line upstream of which the nozzle is convergent, said        internal wall approaching the axis L-L by moving upstream        towards said cross-section and having, in any axial half-plane,        a defined upstream tangent at its intersection with the neck        line,    -   said cowl downstream of said determined abscissa X0 having        indentations in the trailing edge of the cowl which delimit        chevrons distributed in the circumferential direction;    -   said nozzle optionally comprising a central body of revolution        about the axis L-L and surrounded by the cowl, and the radial        passage cross-section at a point on a wall of the cowl being        defined as previously.

Such a nozzle is remarkable in that, in the downstream direction fromsaid determined abscissa (X0) of the neck line:

-   -   the radial passage cross-section of the internal wall of the        cowl increases in an axial half-plane passing through the top of        an indentation,    -   the radial passage cross-section of the external wall of the        cowl decreases in an axial half-plane passing through the top of        an indentation, and    -   the radial passage cross-section of the internal wall of the        cowl decreases in an axial half-plane passing through the tip of        a chevron.

The three conditions verified by the cowl in this nozzle, expressed inradial passage cross-section, refer to the progression of the passagecross-section transversely with respect to the axis L-L, as a functionof the difference between the squares of the radii (R_(exterior)²−R_(interior) ²). They are easier to implement and to verify in thedesign of the nozzle than the conditions expressed at a distancerelative to the tangent to the neck for the cowl alone. Although theyare less representative of the development of the shapes with respect tothe mean flow in the event of pronounced variations in the inclinationof the walls around the axis L-L, they perform similar functions to theconditions defined in the first characterisation of the nozzle, inparticular if the inclination of the walls is not too pronounced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood, and other details, features andadvantages of the present invention will be more clearly apparent fromthe following description given by way of non-limiting example and withreference to the accompanying drawings, in which:

FIG. 1 is a schematic axial section of a convergent-divergent nozzle ofa turbine engine, devoid of anti-noise chevrons;

FIG. 2 is a schematic profile view of the rear of a nozzle cowl definedin accordance with the present invention, which comprises anti-noisechevrons;

FIG. 3 is a superimposition of the schematic cross-sections of the rearof the cowl of FIG. 2, along the axial half-planes passing through thetip of a chevron and through the top of an indentation;

FIG. 4 is an enlarged schematic perspective view of the construction bylines in the axial half-planes of the external and internal surfaces ofthe cowl, in the region of the chevrons and of the indentations;

FIG. 5 is an enlarged schematic perspective view of a cowl thus definedin the region of the chevrons and of the indentations;

FIG. 6 is a superimposition of the schematic cross-sections of the rearof a nozzle according to the invention in the case where the cowlsurrounds a central body of the turbine engine, along the axialhalf-planes passing through the tip of a chevron and through the top ofan indentation, referencing the distances with respect to an axisparallel to the tangent to the neck;

FIG. 7 shows the same geometric configuration as FIG. 6, but referencingthe distances with respect to the axis of the nozzle.

FIRST EMBODIMENT: CIRCULAR CONVERGENT-DIVERGENT REFERENCE NOZZLE

When the cowl does not surround a central body, the rotationallysymmetric cross-section of the nozzle around an axis L-L is defined by across-section perpendicular to the axis L-L. The surface area of across-section is defined solely by the radius of the internal wall ofthe cowl in this plane. Since the walls of the cowl are of circularcross-section, they are convergent or divergent here if the radiusdecreases or increases respectively in the downstream direction.

FIG. 1 shows a known convergent-divergent reference nozzle 1 i of asingle-flow turbine engine. The reference nozzle 1 i comprises anannular reference cowl 2 i having a longitudinal axis L-L and a certainthickness which decreases close to its trailing edge 5 i. The internalwall 3 i of the cowl 2 i defines a nozzle through which an internalgaseous flow F1 passes. The external wall 4 i of this cowl 2 i is incontact with an external gaseous flow F2.

The internal 3 i and external 4 i walls are connected to one another attheir downstream end, with respect to the flow direction of the flows,in order to define the trailing edge 5 i of the reference nozzle 1 iwhich delimits its outlet cross-section 6 i. The internal wall of thecowl 2 i of the reference nozzle 1 i has a convergent part as far as anabscissa X0 on the axis L-L, connected to a divergent part downstream,which defines a neck 7 i at X0. The external wall 4 i of the cowl is,however, simply convergent towards the axis L-L.

The reference nozzle 1 i is devoid of anti-noise chevrons but it issuitable for a turbine engine (not shown) upstream of theconvergent-divergent part shown in FIG. 1. As a function of theconditions on the internal flow which are imposed by this turbine engineupstream of the nozzle and are different according to the flightconditions, the nozzle is designed to correspond in particular to:

-   -   a thrust coefficient at cruising speed CT_(RC), which gives the        ratio between the thrust and the conditions which generate the        internal flow F1 and are created by the turbine engine;    -   a difference CV_DV, between the flow rate coefficient at        take-off speed CD_(CTO) and that at cruising speed CD_(CC),        which is required to be positive in order to increase the        operability of the engine (passing on the good flow in order to        avoid pumping, flotation or overheating).

A cowl 2 defined according to the invention is designed in order toobtain an optimised nozzle 1 which has the same thrust coefficientCT_(RC), the same difference of flow rate coefficient CV_DV, and whichadditionally supplies a gain ΔEPNdB with respect to the intensity of thejet noise of the reference nozzle 1 i. The jet noise can for example bemeasured and/or estimated at a predefined distance from the outletcross-section 6 or 6 i of the relevant nozzle, respectively theoptimised nozzle 1 or the reference nozzle 1 i.

FIG. 2 shows an example of a cowl 2 to the rear of an optimised nozzle 1defined in accordance with the present invention. The shape of theinternal wall 3 of the cowl 2 is identical to that of the internal wall3 i of the cowl 2 i up to the abscissa X0 on the axis L-L, correspondingto the cross-section of neck 7. This cross-section defines a circularneck line 71 on the internal wall 3, as shown on FIG. 4. Moreover, thetangent of the internal wall 3, upstream, in an axial half-plane passingthrough any point on the neck line 71 is substantially parallel to theaxis L-L. By way of example, the shape of the external wall 4 of thecowl 2 is identical to that of the external wall 4 i of the cowl 2 i upto an abscissa X1 on the axis L-L, corresponding to a sectional plane 8upstream of the abscissa X0 of the neck 7. In another embodiment, it ispossible for the zone in which the external wall 4 of the cowl 2 ismodified to be defined differently.

Downstream of the abscissa X0 of the neck 7, the trailing edge 5 of thecowl 2 of the optimised nozzle 1 describes indentations 9 ofsubstantially triangular shape having a rounded top 11. Thus theindentations 9 define anti-noise chevrons 10 also of substantiallytriangular shape and having a rounded tip 13, in the extension of thewall of the cowl 2. Of course, the indentations and the chevrons couldhave any other appropriate shape (for example trapezoidal).

The indentations 9, regularly spaced from one another in thecircumferential direction (although this could be different), aredefined by a base situated on a circular 6 cross-section, limiting thedownstream extension of the cowl, and by a top 11, situated on asectional plane 12 of the abscissa X2, downstream of the neck 7. In thesame way, the chevrons 10 defined by a tip 13 situated on the endcircular cross-section 6 and by a base situated on the intermediatesectional plane 12, are regularly spaced from one another. Furthermore,although it could be otherwise, in the example of FIG. 2, theindentations 9 are identical to one another. Therefore the same appliesto the chevrons 10. Finally, in the example shown, the abscissa X3 ofthe circular cross-section 6 on which the tips 13 of the chevrons arelocated is offset from that of the outlet cross-section 6 i of thereference nozzle 1 i. However, this abscissa forms part of theadjustable parameters in the design of the new nozzle according to theinvention. Therefore an embodiment may be provided in which the circularcross-section 6 would be placed differently.

As FIG. 3 shows, the shape of the cowl 2 according to the invention isdefined so as to:

-   -   firstly, obtain chevrons 10 of which the internal wall is        convergent from the neck 7 and which chevrons re-enter the flux        F1 leaving the nozzle in order to optimise the anti-noise        performance thereof;    -   secondly, on the contrary, allow the possibility for the nozzle        to diverge at the top 11 of the indentations 9 in order to offer        a wider equivalent outlet cross-section to the internal flow F1        and to maintain the operational performance levels of the        nozzle;    -   thirdly, retain a convergent shape of the external wall 4 of the        cowl 2 towards the trailing edge 5 in order to convey the        external flow F2 into the internal flow F1 and promote the        mixing thereof.

In order to achieve this object, first of all the profile of the wallsof the cowl 2 is defined in the axial half-planes passing through theends of the chevrons and of the indentations. These lines are definedwith the aid of splines or any other means known to a person skilled inthe art by imposing a certain number of conditions on said lines.

In an axial half-plane passing through the top 11 of an indentation 9:

-   -   The internal wall 3 of the cowl 2, illustrated in FIG. 4, forms,        between the sectional point 72 of the neck 7 and the internal        point 11A at the top 11 of the indentation, a line 39 determined        by the internal radius R0 of the neck 7, the abscissas X0 of the        point 72 on the neck and X2 of the internal point 11A, the        radius R2 defining the distance of the axis L-L from the point        11A, and an angle α with the axis L-L, imposed on the tangent of        the line 39 at the point 11A.

The external wall 4 of the cowl 2, illustrated in FIG. 4, forms, betweenthe point 82 on the abscissa X1 and the external point 11B at the top 11of the indentation, a line 49 determined by the abscissa X1, the radiusR1 of the external wall at the point 82, the abscissa X2 of the externalpoint 11B (identical to that of the point 11A on the example shown), bythe thickness e of the cowl at the trailing edge 5 (indicated in FIGS. 4and 5), radially separating the points 11A and 11B, and an angle β withthe axis L-L, imposed on the tangent of the line 49 at the point 11B.

In an axial half-plane passing through the tip 13 of a chevron 10:

-   -   The internal wall 3 of the cowl 2 forms, between the sectional        point 73 of the neck 7 and the internal point 13A at the tip 13        of the chevron 10, a line 30 determined by the internal radius        R0 of the neck 7, the abscissas X0 of the point 73 on the neck        and X3 of the internal point 13A, the radius R3 defining the        distance of the axis L-L from the point 13A, and an angle θ with        the axis L-L, imposed on the tangent of the line 30 at the point        13A.    -   The external wall 4 of the cowl 2 forms, between the point 83 on        the abscissa X1 and the external point 13B at the tip 13 of the        chevron 10, a line 40 determined by the abscissa X1, the radius        R1 of the external wall at the point 83, the abscissa X3 of the        external point 13B at the tip of the chevron (equal to that of        the internal point 13A on the example shown), by the thickness e        of the cowl at the trailing edge 5, radially separating the        points 13A and 13B, and an angle γ with the axis L-L, imposed on        the tangent of the line 40 at the point 13B.

Furthermore, the internal radius R2 at the internal point 11A of the topof the indentation 9 is required to be greater than the internal radiusR0 at the neck 7, and the internal radius R3 at the internal point 13Aon the tip of chevrons is required to be less than the same internalradius R0 at the neck 7.

Moreover, the lines 30, 39, 40 and 49 are required to be connectedtangentially to the upstream surfaces of the initial cowl. This impliesin particular that the tangents 70 and 79 of the lines 30 and 39respectively, at the points 73 and 72 on the neck line 71, form an angleof substantially zero with the axis L-L. Furthermore, as FIG. 3 shows,in the axial half-plane these lines are required not to have turningpoints. In particular, the line 30 defining the internal wall 3 betweenthe neck 7 and the tip 13A of a chevron must be constantly convergent,that is to say that its distance from the axis L-L decreases towards thetip 13A. Likewise the angles β and γ are required to be negative, thatis to say that the external face 4 of the cowl converges towards theaxis L-L at the corresponding ends.

These conditions can be reformulated with respect to the mean directionof the internal flow F1 at the neck 7 which is substantially parallel tothe axis L-L, and therefore to the tangent of the internal wall 3 to theneck line 71:

In the downstream direction,

-   -   the re-entrant chevron 10 in the internal flow F1, the sectional        line 30 in the axial half-plane passing through the tip 13        diverges radially towards the interior of the tangent 70 to the        neck,    -   as the indentation 9 is divergent for the internal flow F1, the        sectional line 39 in the axial half-plane passing through the        top 11 diverges radially towards the exterior of the tangent 79        to the neck, and    -   the sectional line 49 in the axial half-plane passing through        the top 11 moves radially closer to the tangent 79 to the neck,        directing the external flow F2 towards the flow F1.

Moreover, a three-dimensional tip-depression line defining the trailingedge 5 is calculated by known means in order to pass through the tops 11of the indentations 9 at the tips 13 of the chevrons 10 according to thechosen shape, the distance of said line from the axis L-L varyingmonotonically, always in the same direction, from the top 11 of anindentation to the tip 13 of a neighbouring chevron 10. In fact, aninternal line 5A and an external line 5B, separated by the thickness e,define this trailing edge respectively for the internal 3 and external 4walls of the cowl. As indicated in FIGS. 4 and 5, in the embodiment theradial thickness of the contour of each indentation 9—defined betweentrailing edge lines 5A and 5B—is kept constant along said trailing edge.Said thickness is preferably equal to the thickness of the trailing edgeof the cowl 2 of the reference nozzle 1, that is to say for example 3.5mm.

As shown in FIG. 4, the surface defining the external wall 4 of the cowltowards the trailing edge 5 is determined by known computer-assisteddesign means, by portions between the indentations and the chevronsbetween the following curves:

-   -   the line 81 defining the external wall of the cowl on the        sectional plane of the abscissa X1, between the points 82 and        83,    -   the trailing edge line 5B between the points 11B and 13B,    -   the line 40 connecting the points 83 and 13B in the axial        half-plane of the chevrons,    -   the line 49 connecting the points 82 and 11B in the axial        half-plane of the indentations.

In addition, it is preferably required that the sectional lines of thissurface through any axial half-plane do not have any turning points.Moreover, a tangent continuity of the connection surfaces with thesurface 4 of the cowl is preferably required upstream of the line 81 atthe connection point. Thus disruptions to the external flow F2 areavoided. Preferably, the tangent of the sectional line of the surfacedefining the external wall in any axial half-plane is required to bealways oriented towards the axis, whilst maintaining the fact that thisexternal surface has no turning point.

As can be seen in FIG. 3, it should be noted that in this surfaceconstruction the axial sectional line 40 passing through the tip 13B ofthe chevron is approximately in the extension of the profile of theinitial nozzle and that the axial sectional line 49 passing through thetop of the indentation corresponds to a recess with respect to thisprofile. In this configuration, the external wall 4 of the cowl remainsentirely inside the surface created by the revolution about the axis L-Lof the line 40 defining its trace in the axial half-plane passingthrough the tip of the chevrons. Thus, the modified cowl does not have,radially, overall dimensions greater than that of the reference nozzle.

The surface defining the internal wall 3 of the cowl 2 towards thetrailing edge 5 is determined between the chevrons and the indentationsin the same way as the external wall 4, by:

-   -   the curve 71 of the cross-section of the internal wall 3 in the        plane of the abscissa X1 between the points 72 and 73,    -   the internal trailing edge line 5A between the points 11A and        13A,    -   the line 30 connecting the points 73 and 13A in the axial        half-plane of the chevrons,    -   the line 39 connecting the points 72 and 11A in the axial        half-plane of the indentations.

Moreover, a tangent continuity, in any axial half-plane, of theconnection surfaces with the internal wall 3 of the cowl is preferablyrequired upstream of the line 71 at the connection point.

It should be noted, as can be seen from FIG. 3, 4 or 5, and from theconditions imposed, that from the tip 13A of the chevron towards the top11A of the indentation, the axial lines of the surfaces defining theinternal wall 3 progressively change orientation: they convergeconstantly without a turning point in the region of the chevron, theydiverge in the region of the indentation. Advantageously, these linesare convex when viewed from the axis L-L around the top 11A of theindentation 9 and their tangent in the region of the trailing edge line5A is divergent. The angle α of the tangent of the axial line 39 ispositive, as is indicated in FIG. 3.

Moreover, as can be seen in FIG. 5, the cowl obtained by assemblingthese surfaces has an azimuthal alternation between convergent angularsectors centred around axial half-planes passing through the tips 13A ofthe chevrons 10, and divergent sectors centred around axial half-planespassing through the tops 11A of the indentations 9.

A brief description of the different steps enabling the definition ofthe optimised nozzle equipped with chevrons 10 is given below.

In a preliminary phase, an initial value is attributed to thedimensional parameters defining the chevrons 10 and the indentations 9towards the trailing edge 5 of the cowl 2, namely the parameters e, X0,X1, X2, X3, R0, R1, R2, R3, θ, γ, β and α.

Then, by any means accessible to a person skilled in the art,performance criteria which are associated with the optimised nozzleequipped with chevrons 10 and designed with the aid of theaforementioned dimensional parameters are calculated. These criteria arefor example the three criteria introduced previously, namely the thrustcoefficient at cruising speed CT_(CC), the difference CV_DV between theflow rate coefficient at take-off speed CD_(CTO) and the flow ratecoefficient at cruising speed CD_(CC), the difference ΔEPNdB between theintensity of the jet noise of the reference nozzle 1 i and that of thenozzle equipped with chevrons according to the invention.

Predefined performance conditions to be satisfied are also associatedwith each criterion, in particular:

-   -   the performance condition associated with the difference between        the thrust coefficients ΔCT is satisfied when this difference        ΔCT is less than a second predefined threshold. Said threshold        is, for example, equal to 0.001 (that is to say ΔCT<0.001);    -   the performance condition associated with the difference between        the flow rate coefficients CV_DV is satisfied when this        difference CV_DV is at least equal to a first predefined        threshold. Said threshold is, for example, equal to 0.015 (that        is to say CV_DV≥0.015); and    -   the performance condition associated with the difference between        intensities of the jet noise ΔEPNdB is satisfied when this        difference ΔEPNdB is positive (that is to say ΔEPNdB>0) and,        preferably, at least equal to a third predefined threshold.

Then an optimisation algorithm is applied to the set of dimensionalparameters thus initialised in order to define the shape of the cowl 2equipped with chevrons, such that the optimised nozzle 1 satisfies thepredefined conditions on said performance criteria.

For example, it is verified that the aforementioned three performancecriteria CV_DV, ΔCT and ΔEPNdB calculated with the initial values of thedimensional parameters each satisfy predefined performance conditionswhich are respectively associated with them.

In the case where at least one of said calculated performance criteriadoes not satisfy the performance condition associated therewith, a newinitial value is attributed to at least one of the aforementioneddimensional parameters, and then the three performance criteria arecalculated again. For example, the new initial value can correspond tothe previous initial value incremented by one unit.

As long as the conditions associated with the three performance criteriaare not satisfied simultaneously, the aforementioned two steps from thepreceding paragraph are repeated.

When the three calculated performance criteria satisfy the associatedperformance conditions, the last values attributed to the dimensionalparameters are validated in order to define the definitive shape of thewalls 3 and 4 of the cowl 2 of the optimised nozzle 1.

SECOND EMBODIMENT: CONVERGENT REFERENCE NOZZLE

In this variant, the reference nozzle 1 i is simply convergent. Anattempt will then be made to reduce the effect of supplementaryconvergence introduced by chevrons.

The cowl 2 of the nozzle obtained according to the present inventionhas, towards its trailing edge 5, a shape comprising chevrons 10 andindentations 9, which is defined by the same parameters as those of thefirst embodiment.

In this case there is no predefined neck, since the reference nozzle isconvergent. A preliminary step therefore consists in choosing across-section 7, for an abscissa X0 on the axis L-L upstream of theoutlet 6 i of the reference nozzle 1 i, on the basis of which the shapeof the internal wall 3 of the cowl 2 of the optimised nozzle ismodified. Once this abscissa X0 is determined, the construction of thesurfaces defining the internal 3 and external 4 walls of the cowl 2 usesthe same parameters and the same method as for the first embodiment.

The method for defining the optimised nozzle therefore comprises apreliminary step compared with the first embodiment which consists inchoosing the abscissa X0 of the neck cross-section of the optimisednozzle. Since the reference nozzle 1 i is convergent, the more thisabscissa X0 is raised in the upstream direction, the larger the neckcross-section 7 is and the more it is possible, as compensation, to makethe chevrons 10 convergent.

The method for defining the optimised nozzle then repeats the steps ofthe method used in the first embodiment.

THIRD EMBODIMENT: ANNULAR REFERENCE NOZZLE

In the case of an exhaust nozzle for a double-flow turbojet engine forexample, the nozzle corresponds to the air conduit defined between theinternal wall of a cowl 2 and the wall 21 of a central body 20 ofcircular cross-section around the axis L-L shown in FIG. 6.

The cross-section of the nozzle is no longer defined by the radius ofthe internal face of the cowl 2. Known methods are used in thisconfiguration and define, as the cross-section, a surface which issubstantially perpendicular to the mean flow, which is no longer a planeperpendicular to the axis L-L. For example, the calculation method knownas “rolling ball” consists in making a ball roll on one of the walls.When this ball touches the two walls, the line which joins the twopoints of contact defines the passage cross-section between these twopoints. By varying the diameter of the ball it is thus possible todefine the passage cross-section over the length of the nozzle. Next,the convergent or divergent nature of the nozzle is characterised as afunction of the evolution of the surface area of this surface as itprogresses downstream.

For the implementation of the invention, the shape of the wall 21 is notmodified. Use is then made of the fact that the wall 21 of the centralbody is a determined surface area having a relatively regular shape, andthat the aim is to characterise portions of wall 3 or 4 of the cowl 2for which the nozzle cross-section is uniformly convergent or divergent.The object of the invention is also repeated, which is that of:

-   -   constructing re-entrant chevrons 10 in the flow F1 coming from        the neck 7,    -   constructing indentations 9 causing divergence of this same flow        F1, and    -   guiding the external flow F2 towards the flow F1 coming from the        neck in the region of the indentations 9.

In a first variant of the invention, the variations in cross-sectionwhich a method of the “rolling ball” type could provide are approximatedby the procedure described below. Reference is made to the meandirection of flow F1 at the neck 7. The tangent of the internal wall 3of the cowl 2 on the neck line 71 in an axial half-plane is no longerobligatorily parallel to this axis.

On the contrary, this tangent substantially follows the mean flow aroundthe central body 20 and its direction therefore represents approximatelythe direction of the internal flow F1 close to the cowl in the axialhalf-plane, in the region of the cross-section of the neck. Therefore,in the definition of the lines 30 and 39 or 40 and 49 defining theinternal 3 and external 4 walls of the cowl 2 in the relevant axialhalf-planes, the direction of the axis L-L is replaced by that of thetangent 70 or 79 defined in this half-plane by the internal wallupstream of the corresponding points 72 or 73 on the neck line 71.

FIG. 6 shows the rear of an optimised nozzle according to the invention,repeating the notations of the first embodiment. The differentcharacteristic locations on the cowl 2, the end of the chevrons 13, thetop 11 of the indentations, the line 71, corresponding to the points 72and 73, of the neck cross-section on the internal wall 3 of the cowl,and the line 81, corresponding to the points 82 and 83, of thecross-section of the external wall of the cowl, are referenced by theirrespective abscissas on the axis L-L: X3, X2, X0, X1.

In contrast, in each axial half-plane, a straight line L′L′ parallel tothe tangents 70 and 79 respectively, and passing through the point X0 ofthe neck 7 is defined. The conditions for the divergence of thecharacteristic points are then determined by their respective distancesfrom this straight line L′L′: D3, D2, D0, D1. Moreover, the angles ofthe tangents are likewise calculated with respect to this straight lineL′L′.

Using these conventions, the same steps defining the walls of the cowl 2as in the first embodiment are repeated but:

-   -   the radius at a point on an internal 3 or external 4 wall of the        cowl 2 is replaced by the divergence at this point from the        straight line L′L′;    -   the angle formed by a tangent in an axial half-plane at a point        on a wall of the cowl with the axis L-L is replaced by the angle        which said tangent forms with the tangent of the internal wall 3        on the neck line 71.

In this way, the condition observed in the first embodiment is apparentin the downstream direction from the abscissa X0:

-   -   the sectional line 30 in the axial half-plane passing through        the tip 13 diverges radially towards the interior of the tangent        70 to the neck,    -   the sectional line 39 in the axial half-plane passing through        the top 11 diverges radially towards the exterior of the tangent        79 to the neck, and    -   the sectional line 49 in the axial half-plane passing through        the top 11 moves radially closer to the tangent 79 to the neck.

The algorithm for defining the cowl then repeats the same steps as inthe first embodiment in order to obtain a nozzle which meets therequired performance criteria.

A variant in this configuration, shown in FIG. 7, consists in referringto the surface area of cross-sections defined in the plane perpendicularto the central axis L-L, even if they are not actually perpendicular tothe mean flow. In this case, the radial passage cross-sectionS′=(R_(extertior) ²−R_(interior) ²) at a point on the walls of the cowl2 is defined as the difference between the square, R_(exterior) ², ofthe distance of this point from the axis L-L, and the square,R_(interior) ², of the distance from the axis L-L of the point on thewall 21 of the central body 20 situated on the same radius perpendicularto the axis L-L, when the central body is present in the sectional planetransverse to the axis L-L containing this point. When the central bodyis not present in the sectional plane transverse to the axis L-Lcontaining a point on a wall of the cowl, the radial passagecross-section is simply the square of the distance of this point fromthe axis L-L.

By applying the convergence and divergence criteria to the radialpassage cross-section, this variant is then transposed directly from thefirst embodiment by replacing the radii R0, R1, R2 and R3 by thecorresponding radial passage cross-sections S′0, S′2, and S′3.

The method can be immediately transposed to the case where the referencenozzle is simply convergent as in the second embodiment.

Finally, the invention also applies to a generalisation of the threeexamples presented, for a nozzle which is not necessarily circulararound the axis L-L. In a variant of the method applied to this case,the surfaces of the cowl are defined by the sectional lines in thesuccessive axial planes by turning azimuthally about a central axis L-L.The parameters defining the indentations 9 and the chevrons 10 in thedifferent axial planes are then different according to the azimuth ofthe axial plane, as a function of the shape required for the transversecross-section of the nozzle around this axis L-L. In contrast, the shapeof the sectional lines of the internal 3 and external 4 walls of thecowl 2 in the axial planes passing through the tops 11 of theindentations 9 and through the tops 13 of chevrons 10 follow theconditions previously set out with respect to the points 72 and 73 ofthe neck line.

The invention claimed is:
 1. A cowl for a nozzle having a longitudinalaxis L-L, the cowl comprising: an external wall and an internal wall,the cowl being delimited radially towards an exterior by the externalwall and, radially towards an interior, by the internal wall, theinternal wall of the cowl having a cross-section with a determinedabscissa on the axis L-L defining a neck line on the internal wall, saidinternal wall converging towards the axis L-L upstream of saidcross-section, wherein an axial plane is defined as a plane passingthrough the L-L axis and an axial half-plane is a part of the axialplane extending on only one side of the axis L-L, and said internal wallhaving, in any of the axial half-plane, a defined upstream tangent atits intersection with the neck line, said cowl having, downstream ofsaid determined abscissa, indentations in the trailing edge of the cowlwhich delimit chevrons distributed in the circumferential direction, inthe downstream direction from said determined abscissa, wherein,considering a first axial half-plane passing through the top of anindentation and a second axial half-plane passing through the tip of achevron: the internal wall of the cowl at said first axial half-planediverges radially towards the exterior with respect to the upstreamtangent of the neck line at locations axially downstream of the neckline to the top of the indentation, to be convex when viewed from theaxis L-L, the external wall of the cowl at said first axial half-planemoves closer to the upstream tangent, at locations axially downstream ofthe neck line to the top of the indentation, and the internal wall ofthe cowl at said second axial half-plane diverges radially towards theinterior with respect to the upstream tangent, at locations downstreamof the neck line to the tip of the chevron, to be concave when viewedfrom the axis L-L.
 2. The cowl for a nozzle according to claim 1,wherein a line defining the external wall of the cowl downstream of saiddetermined abscissa in said second axial half-plane passing through thetip of a chevron is concave when viewed from the axis L-L.
 3. The cowlfor a nozzle according to claim 1, wherein lines defining the externalwall of the cowl in any of the axial half-plane do not have a turningpoint downstream of said determined abscissa of the neck line.
 4. Thecowl for a nozzle according to claim 1, wherein lines defining theinternal wall of the cowl in any of the axial half-plane do not have aturning point downstream of said determined abscissa of the neck line.5. The cowl for a nozzle according to claim 1, wherein a thickness ofthe cowl on the trailing edge is substantially constant.
 6. A nozzle fora turbine engine having the longitudinal axis L-L comprising the cowlaccording to claim
 1. 7. The nozzle according to claim 6, furthercomprising: a central body of revolution about the axis L-L.
 8. Thenozzle according to claim 7, in which a radial passage cross-section ata point on the internal wall of the cowl is defined as a square of aradial distance of said point from the axis L-L minus a square of aradial distance of the axis L-L from a point on a wall of the centralbody situated on the same radius when said central body is present at anabscissa on the axis L-L of said point, and by the square of the radialdistance of said point from the axis L-L when this is not the case, saidnozzle being characterised in that, in the downstream direction fromsaid determined abscissa of the neck line, the radial passagecross-section of the internal wall of the cowl increases in any of theaxial half-plane passing through the top of an indentation.